Fluorescent lamp containing-amalgam-forming material

ABSTRACT

A fluorescent lamp having an electrode mount structure sealed in each end which includes a cathode coil supported on a pair of lead wires. At least one of the mount structures further includes a positive-temperature-coefficient thermistor electrically connected across the lead-in wires, with a quantity of amalgamforming material, such as indium, coated on the surface of the thermistor so as to be heated to a substantially stabilized temperature during lamp operation for regulating the mercury vapor pressure of the lamp substantially independent of the ambient temperature about the lamp.

United States Patent 1191 Latassa et al. Jan. 7, 1975 FLUORESCENT LAMP 3,591,828 7/1971 Washimi et al. 3l3/l78 x CONTAINING AMALGAM FORMING 3,688,l48 8/1972 Fedorenko et al 313/490 MATERIAL P E S f d H G rzmary xammer 1eg r1e nmm [75] Inventors: Frank M. Latassa, Magnol1a;Tad|us Attorney Agent, or Firm Edward Comma T. Sadoskt, Salem, both of Mass.

[73] Assignee: GTE Sylvania Incorporated, [57] ABSTRACT Mass- A fluorescent lamp having an electrode mount struc- [22] Filed: Apr. 8, 1974 ture sealed in each end which includes a cathode coil supported on a pair of lead wires. At least one of the [21] Appl' 458,621 mount structures further includes a positive-temperature-coefficient thermistor electrically connected [52] U.S. Cl. 313/490, 313/180 across the lead-in wires, with a q y of amalgam- [51] Int. Cl. H0lj 61/28 forming materiaL Such as indium, coated m6 53 Field f Search 313 490 174 73 179 face of the thermistor so as to be heated to a substan- 313/1 0; 315/10 tially stabilized temperature during lamp operation for regulating the mercury vapor pressure of the lamp 5 1 R f s Cited substantially independent of the ambient temperature UNITED STATES PATENTS 3,548,241 12/1970 Rasch et al. 313/174 x 8 Claims, 2 Drawing Figures PATENTED JAN 75 FIG.I

BACKGROUND OF THE INVENTION This invention relates to low -pressure mercury vapor discharge lamps and more particularly to fluorescent lamps containing an amalgam-forming material for regulating the internal vapor pressure.

It is well-knownthat the light output of a fluorescent lamp is a function of the mercury vapor pressure, which in turn often depends upon the temperature of the coldest region of the glass envelope of the lamp. It is further known that the envelope cold spot temperature for most efficient lamp operation is approximately 40C, which causes a mercury vapor pressure of approximately 4 to 6 X 10 Torr to occur inside the lamp. Often, due to high lamp loading or high ambient temperatures, the envelope temperature and mercury vapor pressure rise above the optimum value.

Various methods of cooling portions of the lamp envelope to regulate vapor pressure have been employed. Shields have been placed between the electrodes and the ends of the envelope; heat sinks have been attached to the envelope; and the lamp envelope has been increased in size, and made with grooves, depressions and the like. It has also been well-known that mercury vapor pressure may be reduced by the use of an amalgam-forming metal, such as cadmium or indium. U.S. Pat. No. 2,966,602 mentions such an application of an amalgam or mercury at column 4, lines 60-64. It has been observed that the location of the amalgam or amalgam-forming metal in the lamp is an important factor in providing the desired improvement in lamp operation. For example, U.S. Pat. No. 3,007,071 discloses the use of an amalgam-forming metal as a strip or powder located along the length of the tubular lamp envelope where it is not exposed to temperatures much higher than those in the discharge. In a lamp described by U.S. Pat. No. 3,392,298, the mercury pressure is controlled by a coating of indium in the form of a ring at the center of the tubular lamp envelope. Such use of an amalgam-forming metal is suitable for fixing the mercury vapor pressure after the lamp reaches thermal equilibrium, but the mercury pressure in the lamp will be extremely low when the lamp is first started since a considerable time is required for the middle part of the lamp, where-the indium is located, to warm up. As a result the lamp may only emit a third of its normal light output even two or three minutes after being started, and may not emit its full light output until as long a period as twelve minutes has elapsed.

' Accordingly, the use of two sources of amalgam within a fluorescent lamp has been employedone which heats up rather slowly when the lamp is energized, and then controls the mercury vapor pressure during operation, and a secondary source of amalgam which is located closer to the electrodes and thus heats up at a faster rate and provides a sufficient amountof mercury vapor to enable the lamp to reach its output more rapidly. A fluorescent lamp of this type is disclosed in U.S. Pat. No. 3,227,907. According to this patent, the cool spot deposit of amalgam-forming material is provided by a band of indium at the center of the glass tube as described in U.S. Pat. No. 3,392,298, and the hot spot deposit of indium islocated on auxiliary electrodes, such as flag anodes, connected to the cathode coil lead-in wires. Such auxiliary electrodes generally rise quickly in temperatures, and may reach temperatures as high as 300C to 400C. Whatever mercury is-picked up by the indium in this location will be quickly vaporized into the atmosphere of the lamp and quickly diffused through it.

' Although the above-discussed cool spot locations of amalgam, viz., a lengthwise strip or deposit of amalgam-forming metal or a center band of indium, can provide effective pressure regulation, it is difficult to apply the amalgam with sufficient adherence at such locations on the interior surface of the lamp envelope, and'it complicates manufacture of the lamp since portions of the lamp have to be cooled during theexhaust and baking operations in order to prevent the amalgam from melting and flowing away. from the desired location. Further, the use of an amalgam center band or lengthwise strip will block radiation and thus cause an appearance defect and some loss of light output, which may be objectionable in specific applications. Also, amalgam locations toward the center of the lamp envelope are more sensitive to ambient temperature and thereby cause shifts in the mercury control point of the lamp.

According to one approach for overcoming the above-mentioned disadvantages of prior cool spot" locations of amalgam, a strip of the amalgam-forming metal is placed in a wire mesh holder which is wrapped and tied or clamped about the cylindrical portion of the glass mount stem at one or both ends of the lamp envelope. Such a structure is described in the following U.S. Pat. Nos: 3,373,303; 3,422,299; 3,526,802; 3,526,804; 3,534,212; and 3,619,697. The temperature of the amalgam during lamp operation is dictated by the length of the stem and the selected axial position of the mesh wrap along the stem (and thus its distance from the electrode). In addition, however, the amalgam temperature is also affected by the lamp operating temperatures and the ambient temperature about the lamp.

In another approach to the problem, U.S. Pat. No. 3,548,241 describes a method whereby a suitable amalgam-forming metal, such as indium, is heated to the liquid state and then sprayed onto the flared portion of one of the glass mount stems before it is sealed into the envelope. The spray is controlled to deposit a band of the amalgam-forming metal having a thickness of less than microns which extends around the circumference of the flared portion of the stem to provide the cool spot source of amalgam. Starting is facilitated by an auxiliary (hot spot) source of amalgam secured to an electrode cap (disintegration shield). Another reference, viz., U.S. Pat. No. 3,629,641 describes a fluorescent lamp containing amalgams at three different locationsthe main amalgam location is a spray deposit of indium on the stem flare; the hot spot amalgam location is a strip of indium alloy on the electrode cap; and the third and intermediate pressure control location comprises a spot of indium on the stem press. Although providing vapor pressure control over the starting and operating phases of the lamp cycle, the provision of two or more distinct depositions of indium on the lamp mount introduces added complications to the fabrication of the lamp, thereby adversely affecting cost. In addition, difflculty is experienced in obtaining a good indium-to-glass bond, and the temperature of the main body of amalgam-forming material on the glass stem continues to be significantly affected by ambient temperatures.

In view of the temperature sensitivity of prior amalgam-containing lamps, it has been difficult to deter-' mine the proper ratio of amalgam-forming material-to mercury for providing the optimum mercury vapor pressure at different lamp installation sites. If the fixture temperature is substantially hotter or cooler than the ambient for which the lamp is designed, vapor pressure will not be optimum and efficiency and thus light output will be reduced. The use of external heaters and the like have been used to control the amalgam temperature during starting and operation with some success. However, these heating elements consume a significant amount of power, and require temperature sensing control and separate power circuits. These factors detract from the economics, compactness and'reliability of the system.

SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide an improved low-pressure mercury vapor discharge lamp.

A principal object of the invention is to economically provide an electric discharge lamp with an improved mercury-vapor pressure control means.

A particular object is to provide an improved fluorescent lamp containing an amalgam or amalgam forming metal in a manner which optimizes mercury-vapor pressure control over a wide range of ambient temperatures and power loadings.

These and other objects, advantages and features are attained, in accordance with the principles of this invention, by disposing the amalgam-forming material on the surface of a positive-temperature-coefficient thermistor electrically connected across one of the lamp electrodes. The thermistor is self-heated by the current flowing through it when the lamp is ignited and quickly attains a substantially stabilized body temperature during lamp operation. Hence the amalgam-forming metal coated on the thermistor is also heated to this stabilized temperature. As this stabilized temperature is predictable by appropriate selection of the thermistor, the proper ratio of amalgam-forming material and mercury can be chosen to obtain an optimum mercury vapor pressure at the stabilized temperature. Thus, the heated amalgam-forming material coated on the thermistor regulates and optimizes the mercury vapor pressure of the lamp substantially independent of the ambient temperature about the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings in which:

FIG. 1 is an elevational view, partly in section, of a fluorescent lamp embodying the present invention, a portion of the bulb being removed for convenience; and

FIG. 2 is an enlarged perspective view of the mount structure employed in the lamp of FIG. 1, as viewed prior to installation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a fluorescent lamp is shown comprising an elongated tubular glass envelope having the customary coating 11 of phosphor on its inner surface and electrode mounts l2 sealed into each of its ends. The light-transmitting envelope is filled with a small amount of rare gas, such as argon, at low pressure, e.g., one to three Torr, and a small quantity of mercury, say 25 mgs, after which it is hermetically sealed in the usual manner by tipping off the exhaust tube 13 (see FIG. 2) at one or both ends of the lamp.

Each mount structure 12 (FIG. 2) includes the typical reentrant vitreous stem 14 having at one end a flared portion 15, which is sealed about its periphery to the end of the tubular glass envelope, and a press 16 at the inward end supporting a cathode coil electrode 17. The cathode typically consists of a colied tungsten filament carrying the usual alkaline-earth oxide (electronemissive) coating and is supported on a pair of lead wires 18 and 19 sealed through the stem press 16. The lead wires extend to terminal pins 20 and 21 insulatively mounted in the lamp bases 22 attached to each end of the hermetically sealed, light-transmitting envelope 10.

In accordance with the invention, the lamp further includes a positive-temperature-coefficient (PTC) thermistor 23 electrically connected across the cathode coil electrode 17 and having a quantity of amalgamforming material 24 disposed on at least one of its surfaces. For example, as illustrated in FIGS. 1 and 2, the body of thermistor 23, typically a semiconductor composition, may be formed in a disc-type configuration, with a pair of lead wires 25 and 26 electrically attached to opposite flat sides of the disc. The ductile thermistor lead wires 25 and 26 are respectively secured, such as by welding, to the electrode lead wires 18 and 19, to thereby electrically connect the thermistor 23 thereacross, and are shaped to support the thermistor disc as illustrated in a suitably spaced relationship with respect to the cathode coil 17. A selected quantity of an amalgam-forming material 24, e.g., mgs of indium, is coated on the flat surface of the thermistor disc 23 facing away from the cathode coil 17.

In operation, an alternating current is applied across the lamp electrodes sustaining an arc therebetween through the mercury vapor within the envelope, thereby causing the vapor to emit ultraviolet light which excites the phosphor coating 11 to fluorescence, particularly along the major portions of the lamp between the electrodes. During operations at high ambient temperatures or high lamp loadings, the lamp temperature will rise and the mercury vapor pressure increases. If the vapor pressure were uncontrolled, it would soon exceed an optimum value (4 to 6 microns) and light output would drop off.

In accordance with the present invention, however, cathode heater current (supplied by the lamp ballast circuit) flows through the PTC thermistor and thereby heats up the disc body 23 to control the temperature of the amalgam-forming material 24 carried thereon, which in turn regulates the mercury vapor pressure in the lamp substantially independent of the ambient temperature about the lamp.

The PTC thermistor 23 is selected to have a relatively low zero-power (cold) resistance, e.g., 2 to 3 ohms at 25C. When current passes through the device, the resistance increases sharply and the device becomes heated. If the thermistor is connected as a shunt to the cathode coil, the heating of the cathode is relatively unaffected since the resistance of the PTC element rises sharply and rapidly, thus allowing almost full cathode heating current to pass through the lamp coils. With a ambient temperatures. Depending upon the design .pr oper coil design, however, the PTC elementcould be placed in series with the coil. Once the thermistor'is relatively unaffected the ambient temperature to which the lamp is exposed; This heating of the thermistorsubsequently heats the amalgam-forming material ,coated thereon which controlsthe mercury vapor pressure and light output of the lamp over a wide range of.

of f the thermistor and its location. and operating current,-the temperature of the devicewill be relatively constant. The resistance r u ld'not be ifsieftieieiit or advantageous as using it as "a combiried amalgam holder and heater.

Sine-e the PTC device becomes heated rapidly, it will also rapidly heat the amalgam and raise the mercury vapor pressure when the lamp is initially turned on. This will aid lamp starting because amalgam lamps are inherently difficult to start because of the suppressed mercury vapor pressure at typical room ambients.

With the use of the PTC device, manufacture of the amalgam lamp is simplified, with better control of mercury and amalgam-forming metals or compounds. Since. the amalgam .or amalgam-forming metal or com- "pounds are applied directly to the PTC device, the

versus temperaturecharacteristicof the'ty pical PTC' ,7 device shows asomewhat negative slope as the temperature initiallyrisesfromthe cold conditionfThen at a given temperature region, the device ideally switches sharply positive in this region,.the thermistor will have a substantially stabilizedbody temperatureat approxil mately the rated'switching temperature of the device.

A PT C thermistor selected to have a'. switchingl temper-i ature of "approximately 100C is particularly suitable for thepresent application; such a device .will havefa substantially stabilized body temperature of l00 C d uring lamp operation. With the temperature of the device a from a semiconductivestate to that of aninsulatonand f the resistance rises sharplyover a'very narrow tempera ture'range. A selected temperature in this switchingregion isreferred to as the switching .temperatureof the thermiston As the resistance vs. temperature slope-is:

1 problem of the material melting or running during the lamp process is eliminated. When in the prior art the amalgam or amalgam-forming metal or compound is coated onto the glass portions or the lamp, it is exposed to extremely high temperature during the lamp manufacturing'and has a tendency to melt and run from its original loc'ation. v;,

Since the. PTC device is an integralpartof the lamp and receives .its energy -frorn the lamp, ballast-circuit,

the-amalgam lamp can beused where any nonarnalg'am lamp of the same wattage is used. No additional heating apparatus is requiredor external power known, the proper ratio of mercury 'andfatnalgamforming metals or compounds can be chosen'to obtain an optimum mercury vapor pressure 'over a wide range of ambient temperatures. I

not less than 90% over an ambient temperature range from approximately F to 160F, as compared to a non-amalgam 40 watt lamp wherein the 90% relative more than 120C. The PTC amalgam holdermay be at- I tached to one of both lamp mounts, and the total amount of amalgam-forming material on all thermistors in the lamp should have a weight ratio to the mercury in the lamp of from about -2:l to 12:1.

It is obvious that the PTC device can be utilized solely as an internal amalgam heater, but its application v Amy-Pre According to one specific embodiment, the PTC- F holder of amalgam (FIG; 2) is employed at one endiqf a 40 watt fluorescent lamp having an argon fill atv 2.5 Torr and containing25 mgs of mercury. The disc-type PTC thermistor 23 isfsel'ected to have a zero-power ref-Y sistance of 2-3 ohms at 25C, a volting rating of up tog 12 volts AC or -DC and a switching temperatureof, 100C; The'amalgam-forming material24ucornprises a" 7 coatingof about 100 m illigrams'of indium on a flatsurface of the PTC device facing away from the cathode coil 17. The lampreaehed of light output about '60 seconds after ignition, and fprodu'ced a light output ofan hermeti F connected there acro ss',

c and an amalgam-formingmaterial disposed on'at.

necessarythus making the application of'this lamp more universal and lesscostly. Replacement of nonamalgam lamps with this amalgam lamp willfnot be a problem in existing fixtures.- i

. Although theinventionhas beendescribedwith respect' to specific'embodiments, it will be appreciated, that modificationsand changes may be made by those skilled inthe art without de and scope ofthe invention. What-weiclai'mrisr- "containir1g ofrnercuryy.; a. 'Qen'vel pe, 1 at} least 1 one i temperature-coefficient, thermistor electrically {least-forte surface of said thermistor, whereby said mistor during lamp operationffor regulating the mercuryvaporfpressureof said lamp substantially j independent of the ambient temperature about said lamp ' 2. -A lamp according to claim lwherein said electrode comprises a cathode coil. I

' 3. Alamp according to claim2 wherein said cathode .coil is supported-on apair of lead wires sealed through the respective end of saiden velope, and said thermistor is electrically connected acrosssaid pair of lead wires.

4. A lamp according to claim 1 wherein said amalgam-forming material isiindium. l;

5. A lamp according to'claim l: whereinthe total amount of said amalgam-forming material disposed on all thermistors in'said lamp has 'af wei'ght ratio to the mercury in said lampfof from about.2 1 to;l2:l.

6. A lamp according to claim 1 wherein said thermistor is selected to have a substantially stabilized body parting from the truespirit mercury vapordischarge lartrp calily sealed, light-transmitting envelope an inertionizable fill gas anda quantity pair of elctrodes sealed into opposite ends of said i;-

iof said electrodes having a positive- I V l amalgam-forming-material jis heated by said thertemperature of approximately 100C during operation of said lamp.

7. A lamp according to claim 1 wherein said thermistor is of a disc-type configuration, and said amalgam- 8. A lamp according to claim 1 wherein the zeropower resistance of said thermistor at 25C is from about 2 to 3 ohms, said thermistor has a voltage rating of up to 12 volts AC or DC, and the switching temperaforming material is coated on a flat surface of said therture of said thermistor is approximately 100C.

mistor disc. 

1. A low-pressure mercury vapor discharge lamp comprising: an hermetically sealed, light-transmitting envelope containing an inert ionizable fill gas and a quantity of mercury, a pair of electrodes sealed into opposite ends of said envelope, at least one of said electrodes having a positive-temperaturecoefficient thermistor electrically connected thereacross, and an amalgam-forming material disposed on at least one surface of said thermistor, whereby said amalgam-forming material is heated by said thermistor during lamp operation for regulating the mercury vapor pressure of said lamp substantially independent of the ambient temperature about said lamp.
 2. A lamp according to claim 1 wherein said electrode comprises a cathode coil.
 3. A lamp according to claim 2 wherein said cathode coil is supported on a pair of lead wires sealed through the respective end of said envelope, and said thermistor is electrically connected across said pair of lead wires.
 4. A lamp according to claim 1 wherein said amalgam-forming material is indium.
 5. A lamp according to claim 1 wherein the total amount of said amalgam-forming material disposed on all thermistors in said lamp has a weight ratio to the mercury in said lamp of from about 2:1 to 12:1.
 6. A lamp according to claim 1 wherein said thermistor is selected to have a substantially stabilized body temperature of approximately 100* C during operation of said lamp.
 7. A lamp according to claim 1 wherein said thermistor is of a disc-type configuration, and said amalgam-forming material is coated on a flat surface of said thermistor disc.
 8. A lamp according to claim 1 wherein the zero-power resistance of said thermistor at 25* C is from about 2 to 3 ohms, said thermistor has a voltage rating of up to 12 volts AC or DC, and the switching temperature of said thermistor is approximately 100* C. 